In pre-clinical glioma, melanoma, and colorectal cancer models, exogenous melatonin re-polarizes tumour-associated macrophages toward an M1 phenotype, boosts cytotoxic granzyme-B production by CD8+ T cells, and down-regulates programmed-death ligand-1 (PD-L1) on malignant and myeloid cells, thereby amplifying responses to anti-PD-1 therapy (9–11). The gene discussed is GZMB; the disease is neoplasm.